.gamma.-Aminobutyric acid (GABA) is the major inhibitory neurotransmitter in the central nervous system (CNS) (for review see Enna, 1983, Biochem. Pharmacol., 30, 907-15; Enna and Mohler, 1987, Raven Press, New York, 265-79; Lloyd and Morselli, 1987, Medical Biology, 65, (2-3), 159-65; Krogsgaard-Larsen, 1988, Medical Res. Reviews, 8, 1, 27-56; Schwartz, 1988, Biochem. Pharmacol. 27, 3369-76). GABA has been estimated to be present in 60-70% of all synapses within the CNS (Fahn, 1976, Raven Press, New York, 169-83). A reduction in GABA neurotransmission has been implicated in the etiology of a variety of neurological disorders including epilepsy Krogsgaard-Larsen et al., 1988, Medical Res. Reviews, 8, 1, 27-56; Loscher, 1985, Epilepsy and GABA Receptor Agonists: Basic and Therapeutic Research. L. E. R. S. Monograph. Vol. 3, G. Bartholoni, L. Bossi, K. G. Lloyd, P. L. Morselli (Eds.), Raven Press, New York, 109-18); Enna, 1981, Biochem. Pharmacol., 30, 907-14 and Neuropharmacology of Central Nervous System GABA and Behavioral Disorders, G. Palmer (Ed.). Academic Press, New York 1981, 507-25; Rebak et al., 1979, Science, 205, 211-13; Ross and Craig, 1981, J.Neurochem. 36, 1006).
The GABA uptake system has traditionally been classified as either neuronal or glial GABA uptake carriers, on the basis of pharmacological selectivity for specific GABA uptake inhibitors (for review see: Krogsgaard-Larsen, 1988, Medical Res. Reviews, 8, 1, 27-56; Schousboe et al., 1991, GABA Mechanisms in Epilepsy, G. Tunnicliff, B. U. Raess (Eds.) Wiley-Liss, New York, 165-87).
Several investigators (Gaustella et al., 1990, Science, 249, 1303-1306; Clark et al., 1992, Neuron 9, 337-348; Borden et al., 1992, J.Biol. Chem. 267, 21098-21104; Liu et al., 1993, J.Biol.Chem. 268, 2106-2112) have recently cloned, and sequenced, four subtypes of the rat and mouse GABA uptake carrier, whose pharmacology cannot be totally explained by the traditional neuronal and glial GABA uptake carriers. Gaustella et al., (1990, Science, 249, 1303-1306) and Nelson et al. (1990, FEBS Lett. 269, 181-184) reported on the cloning of GAT-1, which appears to be a neuronal GABA uptake carrier due to its high sensitivity to nipecotic acid (Gaustella et al., 1990, Science, 249, 1303-1306), and lipophilic nipecotic acid based compounds and distribution within the central nervous system (CNS) (Radian et al., 1990, J.Neurosci. 10, 1319-1330; Mabjeesh et al., 1992, J.Biol.Chem. 267, 2563-68). GAT-1 is not present outside the CNS (Nelson et al., 1990, FEBS Lett. 269, 181-184; Liu et al., 1992, FEBS. Lett. 305, 110-114). GAT-2 was initially cloned by Lopez-Corruera (1992, J.Biol.Chem. 267, 17491-17493) and is present in the CNS, kidney and liver, and has a pharmacology resembling the glial GABA uptake carrier characterized in primary cell culture. GAT-3 which was initially cloned by Liu et al., (1993, J.Biol.Chem. 267, 2106-2112), appears to be under developmental control, as GAT-3 mRNA is highly expressed in neonatal brain, but weakly expressed in adult brain. GAT-3 is also present in kidney and liver. GAT-4 (Liu et al., 1993, J.Biol.Chem. 268, 2106-2112; also termed GAT-B by Clark et al., (1992, Neuron 9, 337-348) and GAT-3 by Borden et al., (1992, J.Biol.Chem. 267, 21098-21104)), cDNA hybridized only in the CNS, and the mRNA for GAT-4 is highly enriched in the brain stem, but not present in the cerebellum or cerebral cortex. While GAT-4 has been shown to transport .beta.-alanine, it appears to have neuronal localization (Clarke et al., 1992, Neuron 9, 337-348).
The distribution of GAT-1, closely resembles the previously reported distribution of .sup.3 H-Tiagabine receptor autoradiography (Suzdak et al., 1994, Brain Research, 647(2), 231-41), as would be expected due to the high affinity of lipophilic nipecotic acid based GABA uptake inhibitors for the GAT-1 transporter (Clarke et al., 1992, Neuron 9, 337-348). While in situ hybridization has revealed the presence of GAT-4 mRNA in the CNS, there has been no direct demonstration of a discretely localized neuronal GABA uptake carrier, which is not sensitive to lipophilic nipecotic acid based GABA uptake inhibitors.
The inhibition of GABA uptake results in enhanced availability of this inhibitory neurotransmitter in the synaptic cleft and thus to increased GABA'ergic activity. Increased GABA'ergic activity can be useful in the treatment, for example of anxiety, pain and epilepsy, as well as muscular and movement disorders (see, for example, P. Krogsgaard-Larsen et al., Progress in Medicinal Chemistry, 1985, 22, 68-112).
A well-known and potent inhibitor of GABA uptake from the synaptic cleft into presynaptic nerve terminals and glial cells is, for example, 3-piperidinecarboxylic acid (nipecotic acid). However, being a relatively polar compound and therefore unable to cross the blood-brain barrier, 3-piperidinecarboxylic acid itself has found no practical utility as a drug.
In U.S. Pat. No. 4,383,999 and U.S. Pat. No. 4,514,414 and in EP 236342 as well as in EP 231996 some derivatives of N-(4,4-disubstituted-3-butenyl)azaheterocyclic carboxylic acids are claimed as inhibitors of GABA uptake. In EP 342635 and EP 374801, N-substituted azaheterocyclic carboxylic acids in which an oxime ether group and vinyl ether group forms part of the N-substituent respectively are claimed as inhibitors of GABA uptake. Further, in WO 9107389 and WO 9220658, N-substituted azacyclic carboxylic acids are claimed as inhibitors of GABA uptake. EP 221572 claims that 1-aryloxyalkylpyridine-3-carboxylic acids are inhibitors of GABA uptake.
According to Yunger, L. M. et al., J.Pharm.Exp.Ther. 1984, 228, 109, N-(4,4-diphenyl-3-buten-1-yl)nipecotic acid (designated SK&F 89976A), N-(4,4-diphenyl-3-buten-1-yl)guvacine (designated SK&F 100330A), N-(4,4-diphenyl-3-buten-1-yl)-homo-.beta.-proline (designated SK&F 100561) and N-(4-phenyl-4-(2-thienyl)-3-buten-1-yl)nipecotic acid (designated SK&F 100604J) are orally active inhibitors of GABA uptake. These data are summarized in Krogsgaard-Larsen, P. et al., Epilepsy Res. 1987, 1, 77-93.
The above cited references all disclose compounds inhibiting the uptake of GABA via the GAT-1 subtype carrier.
U.S. Pat. No. 4,539,407 discloses .beta.-carboline-3-carboxylate ester derivatives having anticonvulsant activity.
The present invention is directed to identifying novel compounds with affinity for the neuronal subtype of the GABA uptake carrier whose pharmacology resembles that of GAT-4.